1. Field of the Invention
The present invention relates to a semiconductor structure and a method of fabricating the same. In particular, the present invention relates to fabricating bipolar transistors.
2. Description of Related Art
Bipolar transistors are electronic devices with two P-N junctions that are in close proximity to each other. A typical bipolar transistor has three device regions: an emitter, a collector, and a base disposed between the emitter and the collector. Ideally, the two P-N junctions, i.e., the emitter-base and collector-base junctions are in a single layer of semiconductor material separated by a specific distance. Modulation of the current flow in one P-N junction by changing the bias of the nearby junction is called “bipolar-transistor action”.
If the emitter and collector are doped n-type and the base is doped P-type, the device is an “NPN” transistor. Alternatively, if the opposite doping configuration is used, the device is a “PNP” transistor. Because the mobility of minority carriers, i.e., electrons, in the base region of NPN transistors is higher than that of holes in the base of PNP transistors, higher-frequency operation and higher-speed performances can be obtained with NPN transistor devices. Therefore, NPN transistors are the majority of bipolar transistors used to build integrated circuits.
Despite the prevalence of Complementary Metal-Oxide Semiconductor (CMOS) Field Effect Transistors (FET), bipolar transistors have superior device attributes in some areas. This is especially true for analog and power gain applications. Conventional bipolar transistor devices require abrupt emitter to base junctions and well controlled base region lengths. However, they are not formed with circuit density of CMOS structures. Traditional bipolar transistors can have collector current densities approaching 100 mA/μm2. However, useful current densities are almost ten times lower, (around 10 mA/μm2) because of the Kirk effect (also known as base push out). Thus, these traditional bipolar transistors are not suitable for operation in saturation or in reverse-active mode. If such a device is in saturation, the charge stored in base and collector regions and the device becomes very slow. Also, techniques for reducing saturation increases power dissipation.